In recent years, certain luminous extragalactic optical transients have been observed to last only a few days. Their short observed duration implies a different powering mechanism from the most ...common luminous extragalactic transients (supernovae) whose timescale is weeks. Some short-duration transients, most notably AT2018cow, display blue optical colours and bright radio and X-ray emission. Several AT2018cow-like transients have shown hints of a long-lived embedded energy source, such as X-ray variability, prolonged ultraviolet emission, a tentative X-ray quasiperiodic oscillation, and large energies coupled to fast (but subrelativistic) radio-emitting ejecta. Here we report observations of minutes-duration optical flares in the aftermath of an AT2018cow-like transient, AT2022tsd (the "Tasmanian Devil"). The flares occur over a period of months, are highly energetic, and are likely nonthermal, implying that they arise from a near-relativistic outflow or jet. Our observations confirm that in some AT2018cow-like transients the embedded energy source is a compact object, either a magnetar or an accreting black hole.
We present the results from a dense multi-wavelength (optical/UV, near-infrared (IR), and X-ray) follow-up campaign of the nuclear transient AT2017gge, covering a total of 1698 days from the ...transient's discovery. The bolometric lightcurve, the black body temperature and radius, the broad H and He I \(\lambda\)5876 emission lines and their evolution with time, are all consistent with a tidal disruption event (TDE) nature. A soft X-ray flare is detected with a delay of \(\sim\)200 days with respect to the optical/UV peak and it is rapidly followed by the emergence of a broad He II \(\lambda\)4686 and by a number of long-lasting high ionization coronal emission lines. This indicate a clear connection between a TDE flare and the appearance of extreme coronal line emission (ECLEs). An IR echo, resulting from dust re-radiation of the optical/UV TDE light is observed after the X-ray flare and the associated near-IR spectra show a transient broad feature in correspondence of the He I \(\lambda\)10830 and, for the first time in a TDE, a transient high-ionization coronal NIR line (the Fe XIII \(\lambda\)10798) is also detected. The data are well explained by a scenario in which a TDE occurs in a gas and dust rich environment and its optical/UV, soft X-ray, and IR emission have different origins and locations. The optical emission may be produced by stellar debris stream collisions prior to the accretion disk formation, which is instead responsible for the soft X-ray flare, emitted after the end of the circularization process.
We present a spectroscopic analysis of Type I superluminous supernova (SLSN-I), SN 2018bsz. While it closely resembles SLSNe-I, the multi-component H\(\alpha\) line appearing at \(\sim30\) d ...post-maximum is the most atypical. The H\(\alpha\) is characterised by two emission components, one at \(+3000\) km/s and a second at \(-7500\) km/s, with a third, near-zero velocity component appearing after a delay. The blue and central components can be described by Gaussian profiles of intermediate width, but the red component is significantly broader and Lorentzian. The blue component evolves towards lower velocity before fading at \(100\) d post-peak, concurrently with a light curve break. Multi-component profiles are observed in other hydrogen lines including Pa\(\beta\), and in lines of Ca II and He I. Spectropolarimetry obtained before (10.2 d) and after (38.4 d) the appearance of the H lines show a large shift on the Stokes \(Q\) -- \(U\) plane consistent with SN 2018bsz undergoing radical changes in its geometry. Assuming the SN is almost unpolarised at 10.2 d, the continuum polarisation at 38.4 d reaches \(P \sim1.8\%\) implying a highly asymmetric configuration. We propose that the observed evolution of SN 2018bsz can be explained by highly aspherical CSM. After the SN explosion, the CSM is quickly overtaken by the ejecta, but as the photosphere starts to recede, the different CSM regions re-emerge producing the peculiar line profiles. Based on the first appearance of H\(\alpha\), we can constrain the distance of the CSM to be less than \(430\) AU, or even lower (\(<87\) AU) if the pre-peak plateau is related to an eruption that created the CSM. The presence of CSM has been inferred for other SLSNe-I. However, it is not clear whether the rare properties of SN 2018bsz can be generalised for SLSNe-I or whether they are the result of an uncommon evolutionary path, possibly involving a binary companion.
Spectroscopically, TDEs are characterized by broad ( 10\(^{4}\) km/s) emission lines and show large diversity as well as different line profiles. After carefully and consistently performing a series ...of data reduction tasks including host galaxy light subtraction, we present here the first detailed, spectroscopic population study of 16 optical/UV TDEs. We report a time lag between the peaks of the optical light-curves and the peak luminosity of H\(\alpha\) spanning between 7 - 45 days. If interpreted as light-echoes, these lags correspond to distances of 2 - 12 x 10\(^{16}\) cm, one to two orders of magnitudes larger than the estimated blackbody radii (R\(_{\rm BB}\)) of the same TDEs and we discuss the possible origin of this surprisingly large discrepancy. We also report time lags for the peak luminosity of He I \(\lambda\)5876 line; smaller than the ones of H\(\alpha\) for H TDEs and similar or larger for N III Bowen TDEs. We report that N III Bowen TDEs have lower H\(\alpha\) velocity widths compared to the rest of the TDEs in our sample and we also find that a strong X-ray to optical ratio might imply weakening of the line widths. Furthermore, we study the evolution of line luminosities and ratios with respect to their radii (R\(_{\rm BB}\)) and temperatures (T\(_{\rm BB}\)). We find a linear relationship between H\(\alpha\) luminosity and the R\(_{\rm BB}\) and potentially an inverse power-law relation with T\(_{\rm BB}\) leading to weaker H\(\alpha\) emission for T\(_{\rm BB}\) \(\geq\) 25000 K. The He II/He I ratio becomes large at the same temperatures possibly pointing to an ionization effect. The He II/H\(\alpha\) ratio becomes larger as the photospheric radius recedes, implying a stratified photosphere where Helium lies deeper than Hydrogen. We suggest that the large diversity of the spectroscopic features seen in TDEs along with their X-ray properties, can potentially be attributed to viewing angle effects.
We present the photometric and spectroscopic evolution of supernova (SN) 2019cad during the first \(\sim100\) days from explosion. Based on the light curve morphology, we find that SN 2019cad ...resembles the double-peaked type Ib/c SN 2005bf and the type Ic PTF11mnb. Unlike those two objects, SN 2019cad also shows the initial peak in the redder bands. Inspection of the g-band light curve indicates the initial peak is reached in \(\sim8\) days, while the r band peak occurred \(\sim15\) days post-explosion. A second and more prominent peak is reached in all bands at \(\sim45\) days past explosion, followed by and fast decline from \(\sim60\) days. During the first 30 days, the spectra of SN 2019cad show the typical features of a type Ic SN, however, after 40 days, a blue continuum with prominent lines of Si II \({\lambda}6355\) and C II \({\lambda}6580\) is observed again. Comparing the bolometric light curve to hydrodynamical models, we find that SN 2019cad is consistent with a pre-SN mass of 11 M\(_{\odot}\), and an explosion energy of \(3.5\times 10^{51}\) erg. The light curve morphology can be reproduced either by a double-peaked \(^{56}\)Ni distribution with an external component of 0.041 M\(_{\odot}\) and an internal component of 0.3 M\(_{\odot}\) or a double-peaked \(^{56}\)Ni distribution plus magnetar model (P \(\sim11\) ms and B \(\sim26\times 10^{14}\) G). If SN 2019cad were to suffer from significant host reddening (which cannot be ruled out), the \(^{56}\)Ni model would require extreme values, while the magnetar model would still be feasible.
We investigate the thermal emission and extinction from dust associated with the nearby superluminous supernova (SLSN) 2018bsz. Our dataset has daily cadence and simultaneous optical and ...near-infrared coverage up to ~ 100 days, together with late time (+ 1.7 yr) MIR observations. At 230 days after light curve peak the SN is not detected in the optical, but shows a surprisingly strong near-infrared excess, with r - J > 3 mag and r - Ks > 5 mag. The time evolution of the infrared light curve enables us to investigate if the mid-infrared emission is from newly formed dust inside the SN ejecta, from a pre-existing circumstellar envelope, or interstellar material heated by the radiation from the SN. We find the latter two scenarios can be ruled out, and a scenario where new dust is forming in the SN ejecta at epochs > 200 days can self-consistently reproduce the evolution of the SN flux. We can fit the spectral energy distribution well at +230 d with 5 x 10^-4 solar mass of carbon dust, increasing over the following several hundred days to 10^-2 solar mass by +535 d. SN 2018bsz is the first SLSN showing evidence for dust formation within the SN ejecta, and appears to form ten times more dust than normal core-collapse SNe at similar epochs. Together with their preference for low mass, low metallicity host galaxies, we suggest that SLSNe may be a significant contributor to dust formation in the early Universe.
Time-domain astrophysics continues to grow rapidly, with the inception of new surveys drastically increasing data volumes. Democratised, distributed approaches to training sets for machine learning ...classifiers are crucial to make the most of this torrent of discovery -- with citizen science approaches proving effective at meeting these requirements. In this paper, we describe the creation of and the initial results from the \(\textit{Kilonova Seekers}\) citizen science project, built to find transient phenomena from the GOTO telescopes in near real-time. \(\textit{Kilonova Seekers}\) launched in July 2023 and received over 600,000 classifications from approximately 2,000 volunteers over the course of the LIGO-Virgo-KAGRA O4a observing run. During this time, the project has yielded 20 discoveries, generated a `gold-standard' training set of 17,682 detections for augmenting deep-learned classifiers, and measured the performance and biases of Zooniverse volunteers on real-bogus classification. This project will continue throughout the lifetime of GOTO, pushing candidates at ever-greater cadence, and directly facilitate the next-generation classification algorithms currently in development.
This work presents the observations and analysis of ATLAS19dqr/SN 2019bkc, an extraordinary rapidly evolving transient event located in an isolated environment, tens of kiloparsecs from any likely ...host. Its light curves rise to maximum light in \(5-6\) d and then display a decline of \(\Delta m_{15} \sim5\) mag. With such a pronounced decay, it has one of the most rapidly evolving light curves known for a stellar explosion. The early spectra show similarities to normal and `ultra-stripped' type Ic SNe, but the early nebular phase spectra, which were reached just over two weeks after explosion, display prominent calcium lines, marking SN 2019bkc as a Ca-rich transient. The Ca emission lines at this phase show an unprecedented and unexplained blueshift of 10,000 -- 12,000 km/s. Modelling of the light curve and the early spectra suggests that the transient had a low ejecta mass of \(0.2 - 0.4\) M\(_\odot\) and a low kinetic energy of \( (2-4)\times 10^{50}\) erg, giving a specific kinetic energy \(\sim1\) \(10^{51}\) erg/M\(_\odot\). The origin of this event cannot be unambiguously defined. While the abundance distribution used to model the spectra marginally favours a progenitor of white dwarf origin through the tentative identification of \ArII, the specific kinetic energy, which is defined by the explosion mechanism, is found to be more similar to an ultra-stripped core-collapse events. SN 2019bkc adds to the diverse range of physical properties shown by Ca-rich events.
